Polymer sensor

ABSTRACT

A sensor for oxidizing agents contains an oxidizable aromatic polymer. The sensor comprises a piezoelectric crystal coated with a porous or non-porous layer that contains the oxidizable aromatic polymer.

This is a National Stage application of PCT/EP95/04315, which has theinternational filing date of Nov. 3, 1995.

The invention describes a sensor for detecting oxidizing agents, such asoxides of nitrogen (NO_(x)), nitrogen dioxide (NO₂), ozone or per acids,the sensor comprising an oxidizable aromatic polymer, such as apolyarylene ether or polyarylene thioether, as the active component.

It is known (Analyt. Chem., 57(13), 2634-8, 1985), that ozone can bedetected using a piezoelectric sensor coated with 1,4-polybutadiene. Theproduction of the polymer coating by application with a brush presents aproblem in the process described here. The contact surface may bedamaged. Moreover, the homogeneity of the layer may not be reproducibleby this process, which is confirmed by the stated range of frequencychange (2000 to 10000 Hz). Furthermore, the observed frequency changesowing to the ppb quantities of ozone which come into contact are sosmall that they are of the order of magnitude of the noise of thenatural frequency of the piezoelectric crystal (3 to 30 Hz). Inaddition, the reaction of the ozone with the 1,4-polybutadiene resultsin the formation of low molecular weight compounds which may partiallyevaporate. This leads to an opposite change in mass and hence to anerror in the determination of the concentration of the gas.

The detection of NO₂ in the sub-ppm range in a mixture with purenitrogen, in which a dual arrangement of the quartz-SAW components(SAW=surface acoustic wave) having a resonant frequency of 600 MHz isused (M. Rapp et al., Sensors Actuators B 1991, 103-108), has also beendescribed. The coating materials used are ultrathin layers (1 to 15 nm)of lead phthalocyanine and iron phthalocyanine derivatives, applied byvapor deposition or by the Langmuir-Blodgett technique. For example, 15nm thick lead phthalocyanine films permit a limit of detection of 5 ppbfor NO₂ within a response time of a few minutes.

It is also known that electronic frequency generators for generatingoscillations use a piezoelectric element of quartz or PZT ceramic. Oneof the resonant frequencies is selected for the detection of changes inmass and is amplied by the connected external frequency generator, theoscillation in question in the frequency range up to about 20 MHz beingthe fundamental oscillation generated by resonance excitation.

In the case of piezoelectric materials, the following function(Sauerbrey equation) is applicable for the frequency change Δf:

    Δf=-2.3*10.sup.6 *F.sup.2 *Δm/A

in which A is the oscillating surface, F is the fundamental oscillationand Δm is the change in mass. If an oscillating surface (for example aquartz disk) is provided with a coating, the frequency of the sensorsystem changes owing to the increase in mass.

If the coating has absorptive properties, with respect to one or moresubstances in the surrounding medium, the oscillating system reacts witha change in frequency to the resulting absorption. The properties of thesensor (selectivity, sensitivity, regenerability, cumulability) can beadjusted within wide limits by an appropriate choice of the absorber.

However, it should be noted that the oscillation properties of thepiezoelectric materials must not be adversely affected by the coating.Furthermore, the absorber must not react with the substances to bedetected, with formation of volatile substances. Furthermore, rapidreaction with the material to be detected is essential for reasonableuse.

The oscillatory capability of the piezoelectric crystal is generallylost if the applied absorber on the piezoelectric crystal is crystallineor semicrystalline. However, a prediction is never possible. Even whenorganic substances are used, the required properties cannot in generalbe established exactly in a reliable manner. The substance to be used istherefore chosen by a more or less empirical procedure.

It is the object of the invention to avoid the stated disadvantages andto provide a simple and reliable sensor for detecting oxidizing agents,such as ozone, oxides of nitrogen (NO_(x)), nitrogen dioxide (NO₂),hydrogen peroxide and per acids.

By using oxidizable aromatic polymers, such as polyarylene ethers orpolyarylene thioethers, in sensors, it is possible to obtain sensors,for example for ozone, nitrogen dioxide or other strong oxidizingagents, having high resolution and selectivity.

The invention relates to a sensor for oxidizing agents, which comprisesan oxidizable aromatic polymer.

Oxidizable aromatic polymers are aromatic polymers which contain groups,such as sulfide bridges, amino groups, diazo groups, unsaturated bonds,alkyl groups or pendant olefinic groups, which can be oxidized byoxidizing agents.

Preferred oxidizable aromatic polymers are polyarylene ethers orpolyarylene thioethers.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE of the Drawing is a schematic representation of a "flowcell" housing a sensor of this invention, illustrating the use of apreferred type of polymer-coated piezoelectric crystal as a sensor foran oxidizing agent.

Polyarylene ethers are polymers which comprise aromatic units bridged byan oxygen atom. Polyarylene ethers are also referred to as polyaryleneoxides. Polyarylene oxides are described, for example, in Ullman'sEncyclopedia of Industrial Chemistry, 5th Edition, Vol. A21, VCHPublishers, Weinheim 1992, pages 605-614, key word "Poly(PhenyleneOxides), which is hereby incorporated by reference.

Polyarylene thioethers, also referred to as polyarylene sulfides, arepolymers which comprise aromatic units bridged via a sulfur atom.Polyarylene thioethers are described, for example, in Ullmann'sEncyclopedia of Industrial Chemistry, 5th Edition, Vol. A21, VCHPublishers, Weinheim 1992, page 463-471, key word: Polymers,High-Temperature-5. Poly(Phenylene sulfide), which is herebyincorporated by reference. Polyarylene thioethers containing sulfonylgroups and the preparation of said thioethers are described in Chimia 28(1974), 567.

The oxidizable aromatic polymer is also referred to below simply aspolymer. Oxidizing agents are, for example, ozone, nitrogen dioxide(NO₂), oxides of nitrogen (NO_(x)), hydrogen peroxide (H₂ O₂) inorganicor organic peroxides or per acids, such as peracetic acid.

Oxidizable aromatic polymers are, for example, substituted polyaryleneshaving repeating units of the general formula (I)

     (Ar.sup.1).sub.n --X)!.sub.m -- (Ar.sup.2).sub.i --Y)!.sub.j -- (Ar.sup.3).sub.k --Z)!.sub.1 -- (Ar.sup.4).sub.o --W)!.sub.p --(I)

in which Ar¹, Ar², Ar³, Ar⁴, W, X, Y and Z, independently of oneanother, are identical or different. The indices n, m, i, j, k, l, o andp, independently of one another, are zero or integers 1, 2, 3 or 4, andtheir sum must be at least 2; in the formula (I), Ar¹, Ar², Ar³ and Ar⁴are o-substituted and unsubstituted arylene systems having 6 to 18carbon atoms, and W, X, Y and Z are divalent linking groups selectedfrom --SO₂ --, --S--, --SO--, --CO--, --O--, --CO₂ -- and alkylene andalkylidene groups having 1 to 6, preferably 1 to 4, carbon atoms, and atleast one of the linking groups W, X, Y or Z must be an ether bridge.

Preferably used substituents on the aryl ring are CH₃, C₂ H₅, CH(CH₃)₂,C(CH₃)₃, C₆ H₅, OCH₃, Cl, CH₃ C₆ H₅, 3--CH₃ C₅ H₄, 4--CH₃ C₆ H₄,4--(CH₃)₃ C₆ H₄ and 2-naphthyl. In addition to the substituents justmentioned, the remaining hydrogen atoms of the aryl systems may,independently of one another, also be replaced by other substituents,such as halogen or amino, nitro or hydroxyl groups. Block copolymerswhich are composed of units of the formula (I) may also be used.

Preferred polyarylene ethers according to the formula (I) arepolyarylene oxides having repeating units of the formula (II) ##STR1##in which the sum of x and y must be 1 and in which in each case zero<x<1and zero<y<1, and x is zero when y is 1, and vice versa. R¹, R², R³ andR⁴ are selected from hydrogen, CH₃, C₂ H₅, CH(CH₃)₂, C(CH₃)₃, C₆ H₅,OCH₃, Cl, CH₂ C₆ H₅, 3--CH₃ C₆ H₄, 4--CH₃ C₆ H₄, 4--(CH₃)₃ C₆ H₄ and2-naphthyl. R¹ to R⁴ may be identical or different.

Furthermore, polymer blends comprising polyarylene ethers of the formula(II) and polystyrene or polystyrene/styrene mixtures may also be used(Ullmann's Encyclopedia of Ind. Chemistry, Vol. A21, VCH PublishersInc., New York, 1992).

Polyamide/polyarylene oxide or polyolefin/polyarylene oxide blends mayalso be used. The content of the polymers of the formula (II) in theblends is from 5 to 99%, preferably from 10 to 99% and in particularfrom 15 to 99%.

A particularly preferred polyarylene ether is poly-para-2,6-dimethylphenylene oxide! (PPO) having units of the formula (III)##STR2##

(U.S. Pat. No. 3,306,874) or a polymer blend comprising (PPO) andpolystyrene or polystyrene/styrene, which are commercially available.

Preferred polyarylene thioethers are polyphenylene sulfides (PPS) havingthe repeating unit --C₆ H₄ --S--.

Polyarylene sulfide (PAS) or PPS may also contain up to 50 mol percentof a 1,2- and/or a 1,3-link to the aromatic nucleus. PAS or PPS isunderstood as meaning both linear and branched or crosslinked material.Furthermore the PAS or PPS may contain, independently of one another,between 1 and 4 functional groups, e.g. alkyl radicals, halogens orsulfonic acid, amino, nitro, cyano, hydroxyl or carboxyl groups, perarylene unit.

Suitable polymers are in general polymers such as polyarylene ethers orpolyarylene thioethers, each of which has an average molecular weightM_(w) of from 2,000 to 2,000,000, preferably from 10,000 to 500,000, inparticular from 10,000 to 100,000, determined by GPC.

Those crystals of inorganic or organic substances which exhibit thepiezoelectric effect may be used.

Alkaline earth metal titanates, lead/zirconium titanates and quartzes,in particular barium titanate and quartz with the AT section, in whichthe piezoelectric properties have particularly little temperaturedependence, are preferred.

In general, the piezoelectric crystals used have a fundamentaloscillation in a frequency range from 20 kHz to 100 MHz, preferably from0.1 MHz to 50 MHz and in particular from 0.1 MHz to 30 MHz.

If the quartz oscillator is evaluated by the SAW method (SAW=surfaceacoustic wave) (W. Gopel et al.: Sensors--A Comprehensive Survey, VCH,Weinheim, Germany), it is possible to use piezoelectric crystals whosesurface oscillations are in the frequency range from 20 kHz to 1000 MHz.

For example, piezoelectric crystals which are provided with a coatingwhich comprises at least one oxidizable aromatic polymer are suitable asa sensor.

The polymer used or the polymer blend can be applied to one or bothsides of the piezoelectric crystals by general coating methods. Coatingmethods based on polymer or monomer solutions, for example spin coating,dip coating or spray methods, are preferred. All organic substanceswhich dissolve the respective polymer or monomer in a definedtemperature range are suitable. Polyarylene ethers are dissolved, forexample, in chloroform. For example, caprolactam, 2,4,6-trichlorophenol,preferably isoquinoline, 1-methoxynaphthalene and 1-chloronaphthaleneare suitable for dissolving polyarylene sulfides. When a monomersolution is used, the polymerization can be carried out by generalsurface polymerization techniques, such as laser induction or atemperature increase.

The adhesion of the coating to the sensor surface can be improved byapplying an adhesive intermediate layer. The adhesive intermediate layerconsists of or contains a polymer having pendant olefinic groups, suchas polybutadiene or polyisoprene, polyacrylate, polymethacrylate orpolystyrene.

According to the invention, the aftertreatment of the applied polymerlayer is effected by drying in commercial drying units, in air, in inertgas or under reduced pressure, at temperatures of 0 to 350° C.,preferably 30 to 300° C. and in particular 50 to 300° C. It is alsopossible to repeat a plurality of coating and drying steps iterativelyto achieve thicker polymer layers.

After drying, the amount of coating material on the piezoelectriccrystal used is 1 ng/cm² to 1 mg/cm², preferably 5 ng/cm² to 10 mg/cm²and in particular 10 ng/cm² to 2 mg/cm².

The piezoelectric sensor is exposed to the gas to be tested, in a flowcell having a defined volume flow. The sensor frequency is eitherevaluated directly or is mixed with a stablilized reference frequencyand then evaluated (plot of the frequency or of the frequency changeagainst time). By means of downstream processors, the signal change canbe converted directly into mass changes and visualized on a display.

The reaction of the sensor to nitric oxide (NO) is small. However, itcan be improved if the gas stream to be investigated is passed, beforepassage over the sensor, through an oxidative inorganic or organiccompound which has a redox potential of at least 0.96 V against astandard hydrogen electrode (SHE), for example chloride of lime, sodiumhypochlorite, vanadium pentoxide or dichlorodicyanoquinone. Theseconvert the NO into NO₂, to which the sensor reacts with highresolution.

It is also possible to determine NO and NO₂ alongside one another in thegas mixture by measuring the gas stream on the one hand with the use ofpreliminary oxidation (measurement of the sum of NO₂ and the NO₂ formedfrom NO) and, on the other hand, without preliminary oxidation(measurement of the NO₂ without reaction of the NO). The differencebetween the two measurements gives the respective amounts of NO and NO₂in the gas mixture.

The invention also relates generally to a gas sensor having increasedsensitivity and a long life, comprising a piezoelectric crystal in whichthe surface of the crystal is provided with a porous polymer, a processfor its preparation and the use of the sensor for detecting ozone oroxides of nitrogen (NO_(x)). The porous polymer layers may also be usedas an active component in sensors operating according to anotherprinciple.

By using a porous polymer layer in sensors, a higher sensitivity andlonger life of the sensors are obtained.

A porous polymer layer or coating can be produced by two methods. Thisis described by way of example for a quartz oscillator.

Coating method a.)

In a first step, the quartz oscillator is immersed in a solution of thepolymer used for the gas analysis and then, in a second step, in anonsolvent, until substantially all solvent has been replaced bynonsolvent. The nonsolvent is then removed by drying.

It is possible to use all solvents which dissolve the polymer. Suitablenonsolvents are preferably substances which are readily miscible withthe solvent. This results in rapid replacement of the solvent by thenonsolvent (phase inversion), which leads to very porous surfaces. Apreferred solvent/nonsolvent combination is, for example,N-methylpyrrolidone/water or tetrahydrofuran/acetone.

Coating method b.)

In a first coating step, the quartz oscillator is immersed in a solutionof a polymer which has a strong adhesion effect. After the quartz hasbeen dried, it is immersed, in a second coating step, in suspension ofporous particles of the oxidizable aromatic polymer. The quartz is thendried again. Here, it is advantageous to increase the temperature towardthe end of the drying--for example to about 10° C. above the glasstransition temperature of the polymer of the adhesive intermediatelayer--in order to achieve good adhesion of the polymer particles to thepolymer of the adhesive intermediate layer, which polymer was applied inthe first step. Substances which may be used for the adhesiveintermediate layer are all polymers which exhibit good adhesion to thequartz surface. Preferred polymers are those which can be heated abovetheir glass transition temperature in the second drying step, so thatgood bonding of the porous particles on the quartz oscillator isensured. Polysiloxane, polyacrylate, polymethacrylate, polystyrene,polyisoprene or polybutadiene may preferably be used. Polymers whichalso undergo thermal crosslinking on heating and thus result inparticularly good adhesion of the porous particles, such aspolybutadiene or polyisoprene, are particularly suitable.

The preparation of the porous particles used is described, for example,in German Patent Application P 44 39 478.0, entitled "Filtermaterial aufPolymerbasis zur Entfernung von Komponenten aus Gasen und Flussigkeiten"Polymer-based filter material for removing components from gases andliquids!, filed on Nov. 8, 1994, which corresponds substantially to U.S.patent Ser. No. 08/836,221, filed Aug. 4, 1997, and which is herebyincorporated by reference.

Porous or nonporous layers of the oxidizable aromatic polymers areparticularly suitable for the preparation of sensors for ozone ornitrogen dioxide.

Sensors according to the invention can be used, for example, in the areaof work safety, in immission and emission measurements and as filtermonitors.

The sensors according to the invention operate in a wide temperaturerange. In general, the temperature range is from -10 to 100° C., inparticular from -10 to 50° C. Higher operating temperatures arepossible. Thermostating of the sensors is not required.

Turning now to the Drawing, a "flow cell" 10 houses a sensor 20 of thisinvention, comprising a piezoelectric crystal 11 coated with anintermediate coating (such as a polybutadiene coating) 12 to which isadhered an outer coating 13 comprising an oxidizable aromatic polymer.In a preferred embodiment, the outer coating 13 is porous (since itcontains porous particles). A gas stream to be tested 15 is allowed toflow over the sensor 20, resulting in a frequency change which is testeddirectly with oscillator 17.

EXAMPLES

1) Commercial HC-18U quartzes (fundamental frequency: 11.5 MHz) wereunsoldered from their protective housing and immersed in a 1% strengthsolution of PPO in chloroform. The sensor was then dried at 70° C. underreduced pressure for 5 hours. The oscillation capability of the coatedsensor was tested using a transistorized oscillator, which permits thequartz oscillators to oscillate at from 0.1 to 30 MHz in parallelresonance, and a 10 MHz frequency counter (resolution 0.1 Hz) with aconnectable input attenuator and thermostated gating.

Coating with PPO: 32.9 μg (9398 Hz)

NO₂ concentration: 600 ppm of NO₂ in helium

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time  min!          0   1  4   5  8   9  12  16 20  21    Δf  Hz!          0   2  6   14 60  80 124 168                                      202 210    __________________________________________________________________________    Time  min!          24  29 33  37 45  49 57  72 92  132    Δf  Hz!          230 258                 282 300                        336 352                               380 426                                      476 558    __________________________________________________________________________

2) Example 1 was repeated with the following characteristics:

Coating with PPO: 111.2 μg (31764 Hz)

NO₂ concentration: 600 ppm with NO₂ in helium

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time  min!          0   1  4   5  8   9  12  16 20  21    Δf  Hz!          0   2  4   32 194 236                               302 362                                      405 416    __________________________________________________________________________    Time  min!          24  29 33  37 45  49 57  72 92  132    Δf  Hz!          436 468                 488 504                        532 542                               564 598                                      638 700    __________________________________________________________________________

3) Example 1 was repeated with the following characteristics:

Coating with PPO: 18.2 μg (5196 Hz)

O₃ concentration: 100 ppb in air

Flow rate: ˜100 l/h

    ______________________________________    Time  0      21     30   44   60   120  155  238  380     min!    Δf  Hz!          0      2      4    6    10   20   26   38   60    ______________________________________    Time  470    600    680  770  830  905  1010     min!    Δf  Hz!          70     82     90   98   104  110  118    ______________________________________

The examples show that both NO₂ and ozone are detected virtuallylinearly by a sensor which contains a polyarylene ether.

4) Commercial HC-18 U quartz (fundamental frequency: 11.5 MHz) wereunsoldered from their protective housing and immersed in a 1% strengthsolution of PPS in isoquinoline. The sensor was then dried at 70° C.under reduced pressure for 5 hours, coating with PPS being 21.1 μg. Theoscillation capability of the coated sensor was tested with atransistorized oscillator, which permits quartz oscillators to oscillateat from 0.1 to 30 MHz in parallel resonance, and a 10 MHz frequencycounter (resolution 0.1 Hz) with a connectable input attenuator andthermostated gating. The sensor was brought into contact with an NO₂-containing gas stream. NO₂ concentration: 600 ppm of NO₂ in helium,flow rate: ˜100 l/h

    __________________________________________________________________________    Time  min!          0   20 25  31 37  41 47  51 58  63    Δf  Hz!          0   74 84  90 100 112                               120 126                                      140 144    __________________________________________________________________________    Time  min!          68  78 88  98 108 128                               158 188                                      218 267    Δf  Hz!          152 162                 178 196                        208 240                               274 304                                      330 450    __________________________________________________________________________

The example shows that NO₂ is virtually linearly detectable by thesensor which contains a sulfur-containing polymer.

5) Sensor with porous coating according to Method a.) Commercialquartzes were removed from their protective housing and immersed in a 1%strength solution (sensor 1) and in a 5% strength solution (sensor 2) ofPPO in N-methylpyrrolidone (NMP). The quartzes, to whose surface a filmof NMP solution adheres, were then immersed directly in distilled water.After one minute, the quartz was removed again and dried, theoscillation capability was then checked and the new oscillationfrequency was measured. The mass of the coating was determined from thefrequency difference using the Sauerbrey equation. (cf. page 2). Anozone-containing gas stream was allowed to flow over the quartzes in acell. The frequency change was tested directly with a transistorizedoscillator, which permits quartz oscillators to oscillate between 0.1and 30 MHz in parallel resonance, and a 10 MHz frequency counter with aconnectable input attenuator and thermostated gating.

Example 5.1 Sensor with Porous Layer

Coating with PPO: 19.23 μg

Ozone concentration: 500 ppb

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time       200          450             600                750                   840                      900                         1200                            1500                               1800                                  2100                                     2520                                        3000                                           3300                                              3600                                                 3900     min!    Δf       84 132             180                228                   264                      284                         368                            476                               568                                  672                                     812                                        976                                           108                                              1164                                                 1276     Hz!    __________________________________________________________________________

Example 5.2 Sensor with Porous Layer

Coating with PPO: 117.83 μg

Ozone concentration: 500 ppb

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time       200          450             600                750                   840                      900                         1200                            1500                               1800                                  2100                                     2520                                        3000                                           3300                                              3600                                                 3900     min!    Δf       304          640             756                932                   1004                      1048                         1268                            1428                               1572                                  1740                                     1968                                        2216                                           2344                                              2468                                                 2588     Hz!    __________________________________________________________________________

6) Sensor with porous particles according to Method b.) Commercialquartzes were removed from their protective housing and immersed in a 1%strength solution of polybutadiene in toluene. The quartzes were thendried and the frequency change measured. The sensors provided with thethin polybutadiene layer were then immersed at 25° C. in a 1% strengthsuspension of PPS and 1-methoxynaphthalene and the quartzes were driedagain. After evaporation of the solvent, the temperature was increasedto 100° C. for about 30 minutes and the frequency change was measuredagain.

An ozone-containing gas stream was allowed to flow over the coatedquartzes in a cell. The frequency change was tested directly with atransistorized oscillator, which permitted quartz oscillators tooscillate between 0.1 and 30 MHz in parallel resonance, and a 10 MHzfrequency counter with a connectable input attenuator and thermostatedgating.

Example 6.1 Sensor with Porous Particles

Coating with polybutadiene: 4.3 μg

Coating with PPS: 35.16 μg

Ozone concentration: 100 ppb

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time       0 3 8 11               19                 42 70 98 120                             149                                190                                   213                                      254                                         284                                            301     min!    Δf       0 4 20             28               54                 118                    188                       242                          282                             332                                402                                   448                                      494                                         522                                            536     Hz!    __________________________________________________________________________

Example 6.2 Sensor with Porous Particles

Coating with polybutadiene: 2.26 μg

Coating with PPS: 33.74 μg

Ozone concentration: 200 ppb

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time       0 3 4 5 7 10                   13 15 20 25 31 40 45 46 47     min!    Δf       0 20           35             50               72                 96                   132                      144                         168                            196                               220                                  248                                     256                                        262                                           270     Hz!    __________________________________________________________________________

Example 6.3 Sensor with Porous Particles

Coating with polybutadiene: 1.92 μg

Coating with PPS: 20.82 μg

Ozone concentration: 1 ppm

Flow rate: ˜100 l/h

    __________________________________________________________________________    Time       0 2  3  5  7 10 12 15 17 20 25 30 40 50 60     min!    Δf       0 76           142              220                 304                    380                       428                          492                             520                                554                                   596                                      630                                         678                                            712                                               734     Hz!    __________________________________________________________________________

The examples described above show that an increasing frequency changecan be observed with increasing concentration, and even concentrationsof 100 ppb (Example 6.1) can still be readily measured. A very long lifeis also found (Examples 5.1 and 5.2: 3900 min).

We claim:
 1. A sensor for oxidating agents, which comprises apiezoelectric crystal having a plurality of layers on a surface thereof,including an intermediate layer and an outer layer, said outer layercomprising at least one oxidizable aromatic polymer.
 2. The sensor asclaimed in claim 1, wherein said outer layer comprises porous oxidizablearomatic polymer particles.
 3. The sensor as claimed in claim 1 whereina said intermediate layer comprises a polymer having pendant olefinicgroups or polybutadiene, polyisoprene, polyacrylate, polymethacrylate,polystyrene or polysiloxane.
 4. The sensor as claimed in claim 1,wherein said piezoelectric crystal is an alkaline earth metal titanatecrystal, a lead zirconium titanate crystal or a quartz crystal.
 5. Thesensor as claimed in claim 1, wherein a said oxidizable aromatic polymeris a substituted polyarylene having repeating units of the generalformula (I)

     (Ar.sup.1).sub.n --X!.sub.m -- (Ar.sup.2).sub.i --Y!.sub.j -- (Ar.sup.3).sub.k --Z!.sub.1 -- (Ar.sup.4).sub.o --W!.sub.p --(I)

in which Ar¹, Ar², Ar³, Ar⁴, W, X, Y and Z, independently of oneanother, are identical or different; Ar¹, Ar², Ar³ and Ar⁴ areo-substituted or unsubstituted arylene groups having 6 to 18 carbonatoms, and W, X, Y and Z are --SO₂ --, --S--, --SO--, --CO--, --O--,--CO₂ -- or alkylene or alkylidene having 1 to 6 carbon atoms; and theindices n, m, i, j, k, l, o and p, independently of one another, arezero or a number from 1 to 4, and their sum must be at least
 2. 6. Thesensor as claimed in claim 5, wherein at least one of the groups W, X, Yor Z is --O-- or --S--.
 7. The sensor as claimed in claim 5, wherein thelayer comprising at least one oxidizable aromatic polymer comprises aporous coating.
 8. The sensor as claimed in claim 5, wherein a saidoxidizable aromatic polymer is oxidized, sufficiently for gas analysispurposes, by ozone, an oxide of nitrogen, a peroxide or a per acid. 9.The sensor as claimed in claim 1, wherein said oxidizable aromaticpolymer is a sulfur-containing polymer or a polyarylene ether.
 10. Thesensor as claimed in claim 9, wherein said sulfur-containing polymer isa polyarylene thioether.
 11. The sensor as claimed in claim 10, whereinsaid polyarylene thioether is a linear or branched polyphenylenesulfide.
 12. The sensor as claimed in claim 9, wherein said polyaryleneether comprises poly-p-(2,6-dimethylphenylene oxide).
 13. A gas analysisdevice comprising a flow cell containing gas sensor as claimed in claim1 and an oscillator in a direct testing relationship with said gassensor.
 14. A method for analyzing for an oxidizing agent comprising thestep of bringing the oxidizing agent into contact with a sensor asclaimed in claim
 1. 15. The method as claimed in claim 14, wherein theoxidizing agent is ozone, a nitrogen oxide, a peroxide or a per acid.16. A process for the preparation of a gas sensor for gas analysis froma piezoelectric crystal, said process comprising:a) immersing thecrystal in a solution containing a oxidizing agent-sensing polymer and asolvent therefor; b) immersing the thus-immersed crystal in a nonsolventfor said oxidizing agent-sensing polymer until substantially all of saidsolvent has been replaced by said nonsolvent, and c) removing saidnonsolvent by drying such that said oxidizing agent-sensing polymerremains on the surface of the crystal in porous form.
 17. A process forthe preparation of a gas sensor for gas analysis from a piezoelectriccrystal, said process comprising.:a) immersing the crystal in a solutioncontaining a polymer having an adhesive action, b) drying thethus-immersed crystal, c) immersing the thus-dried crystal in asuspension containing porous particles of an oxidizing agent-sensingpolymer, and d) drying the thus-immersed crystal until said porousparticles are bonded to surfaces of the crystal by an intermediate layercomprising said polymer having an adhesive action.
 18. The process asclaimed in claim 17, wherein said polymer having an adhesive action ispolybutadiene, polyisoprene, polyacrylate, polymethacrylate, polystyreneor polysiloxane.